Machine Design Questions and Answers – Viscosity

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This set of Machine Design Multiple Choice Questions & Answers (MCQs) focuses on “Viscosity”.

1. Viscosity is defined as the external resistance offered by a fluid to change its shape or relative motion of its parts.
a) Yes
b) It is internal resisting force
c) It is not offered but exerted on the fluids
d) None of the listed

Explanation: It is an internal resisting force.

2. Stream line flow happens when intermediate layers move with velocities proportional to the square of distance from the stationary plate.
a) True
b) False

Explanation: It is proportional to distance and not square of it.

3. Newton law of viscosity states that shear stress is proportional to rate of shear at any point in the fluid.
a) True
b) False

Explanation: P/A proportional to U/h.
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4. Calculate the kinematic viscosity if Saybolt viscosity is 400cSt.
a) 400SUS
b) 40.25SUS
c) 86.32SUS
d) 87.55SUS

Explanation: z=0.22t-[180/t] where t=400.

5. Viscosity of lubricating oil decrease with increasing temperature.
a) Yes
b) It increases linearly
c) It increases hyperbolically
d) it remains constant

Explanation: Intermolecular forces decrease on the increase of temperature.

6. Which of the following lubricant has least rate of change of viscosity w.r.t temperature.
a) VI=20
b) VI=30
c) VI=40
d) VI=50

Explanation: Greater the VI, lesser is the rate of change w.r.t temperature.

7. Which of the following are not true for petroff’s equation?
a) Shaft is considered concentric with the bearing
b) Bearing is subjected to light load
c) Is used to find coefficient of friction
d) Frictional torque is given by fpr²l

Explanation: M=fWr=f(2prl)r, W=projected area of bearing x pressure.

8. In hydrodynamic lubrication, film thickness remains unaffected by change in speeds.
a) True
b) Increase with increase in speed
c) Decrease with increase in speed
d) Disappear as the speed tends to infinty

Explanation: As speed increases more and more lubricant is forces and pressure builds up thus separating the two surfaces. There is transition from thin film thick film.

9. For a hydrostatic thrust bearing,
Thrust load=450kN, shaft speed=730rpm, shaft diameter=450mm, recess diameter=310mm, film thickness=0.15mm, viscosity of lubricant=160SUS and specific gravity=0.86.
Calculate supply pressure
a) 10.2Pa
b) 4.01Pa
c) 4.01Mpa
d) 10.2Mpa

Explanation: P=2Wln(225/155)/[π(225²-155²)] N/mm².

10. For a hydrostatic thrust bearing,
Thrust load=450kN, shaft speed=730rpm, shaft diameter=450mm, recess diameter=310mm, film thickness=0.15mm, viscosity of lubricant=160SUS and specific gravity=0.86.
Calculate flow requirement
a) 0.89l/s
b) 38.94l/min
c) 28.8l/min
d) None of the mentioned

Explanation: Q=πPhᵌ/6µln(225/155) whereµ=z/10⁹ and z=0.86x[0.22×160-180/160]. µ=29.3 x 10¯⁹N-s/mm².

11. For a hydrostatic thrust bearing,
Thrust load=450kN, shaft speed=730rpm, shaft diameter=450mm, recess diameter=310mm,film thickness=0.15mm,viscosity of lubricant=160SUS and specific gravity=0.86.
Calculate power loss in pumping.
a) 2.68kW
b) 3.35kW
c) 2.6kW
d) 4.2kW

Explanation: kW=Q(P-0)x10¯⁶.

12. For a hydrostatic thrust bearing, Thrust load=450kN, shaft speed=730rpm, shaft diameter=450mm, recess diameter=310mm,film thickness=0.15mm,viscosity of lubricant=160SUS and specific gravity=0.86.
Calculate frictional power loss.
a) None of the listed
b) 2.3kW
c) 3.56kW
d) 4.2kW